Fixed #515.

Thanks to @HyroVitalyProtago for the suggestion! Empty string was definitely a weird enum, and so I do feel like it's appropriate to change it to a more representative string. I had suggested neutral in the issue, but upon thinking it through further I feel like @HyroVitalyProtago's original suggestion of none is more appropriate. This is because the input in question may not actually be associated with any hand at all, such as is the case with gaze input, whereas neutral implies to me that the input is actually associated with a hand but doesn't know or care which. i.e: A Vive wand that hasn't established it's spatial relationship yet strikes me as a "neutral" input device, while a cardboard device has no hand association whatsoever. In both cases none feels like an appropriate descriptor of the handedness.